There are different systems for the electronic detection of items based on magnetic phenomena, which particularly comprehend tags that can be activated/deactivated and their manufacturing method, the detector thereof and the system of activating/deactivating said tags.
The magnetic tag, object of the present invention, can be used in this type of systems and is based on magnetic microwires obtained by the Taylor process.
The Taylor process is known for the manufacturing of microwires that allows obtaining microwires with very small diameters, comprised between one and various tenths of a micrometer, by a simple process. The microwires thus obtained can be made from a great variety of magnetic and non-magnetic alloys and metals. This process is described, for example, in the article “The Preparation, Properties and Applications of some Glass Coated Metal Filaments Prepared by the Taylor-Wire Process” W. Donald et al., Journal of Material Science, 31, 1996, pp 1139-1148.
The most important characteristic of the Taylor method or process is that it allows obtaining metals and alloys in the form of a microwire with insulating sheath in a single simple operation, which entails a cost-reduction in the manufacturing process.
The process for obtaining magnetic microwires with insulating sheath and amorphous microstructure is described, for example, in the article “Magnetic Properties of Amorphous Fe—P Alloys Containing Ga, Ge and As” H. Wesner and J. Schneider, Stat. Sol. (a) 26, 71 (1974), Phys. Stat. Sol. (a) 26, 71 (1974).
The properties of magnetic amorphous microwire with insulating sheath, related to the object of the present invention, are described in the article “Amorphous glass-covered magnetic wires: preparation, properties, applications”, H. Chiriac, T A Óvári 1997 In: Progress in Materials Science, Elsevier Science Ltd. Great Britain, Vol 40, pp. 333-407.
The alloys used in the manufacturing of the microwire core are of the transition metal metalloid type and have an amorphous microstructure. The influence of the geometry of the microwire on its magnetic behaviour is due to the magnetoelastic character of the alloys used that, in turn, depend on the magnetostriction constant thereof.
Systems for detecting items based on magnetic materials are well known. The Picard patent (French patent FR-763,681) shows the first device of this type. The described device is based on the use of a Permalloy-type soft magnetic material tape that, when subjected to an alternating magnetic field, induces harmonics in a detector which are clearly different from those originated by other types of metals.
Ever since Picard filed his patent, there have been great efforts to improve tags from the point of view of their size, as well as their detectability at a distance from the receiver and the possibility of activating and deactivating them. The greater part of the effort has been centered on finding materials with lower coercive forces and greater permeability than permalloy. As the voltage pulse generated in the detector due to the presence of the tag depends on the characteristics of the hysteresis cycle of the metal used, the attempt has always been made to find materials with low coercive force and high permeability in order to obtain higher order harmonics, and with a higher amplitude, for lower values of the applied field, thus making the tag easier to distinguish.
Amorphous magnetic materials in the form of tape have low coercive forces and high susceptibilities that can be optimized to be used in electronic equipment for detecting items by means of suitable heat treatments in the presence or absence of a magnetic field. Thus, for example, U.S. Pat. No. 6,475,303 refers to the use of compositions based on CoNiFeSiBC.
There are other materials that have clear advantages from the detection point of view. These are amorphous materials having magnetic bistability in their hysteresis cycles. This phenomenon is related to the occurrence of a Barkhausen jump in the hysteresis cycle of the material for a certain value of the applied magnetic field. The material has a remanence magnetization value that is not zero for a zero field. To reverse this magnetization, it is necessary to apply a magnetic field in the opposite direction. The critical field is the minimum field necessary to achieve the magnetization reversal. This behaviour is fundamentally found in wires. (The magnetization reversal in amorphous wires. M. Vázquez, D. X. Chen 1995 IEEE Trans. Magn. 31, 1229-1238) and in amorphous magnetic microwires with a high longitudinal anisotropy due to their high magnetostriction constant (Magnetic Properties of glass-coated amorphous and nanocrystalline wires, M. Vázquez, A. P. Zhukov 1996, J. Magn. Magn Mat. 160, 223-228).
When a bistable magnetic material is used in a detection system, the pulse detected due to its presence is substantially independent of the variation rhythm of the magnetizing field and of the intensity thereof, as long as this intensity exceeds a minimum threshold value.
U.S. Pat. No. 4,660,025 discloses a detection system in which a bistable amorphous magnetic wire with a minimum length of 7.6 cm is used as a tag. In this case, an alternating magnetic field is applied to a certain area of space and an alarm is activated when a disturbance is detected in said magnetic field. This happens when a tag is introduced in this area and the magnetic field value exceeds the critical field of the wire, producing a magnetization reversal. This is known as “snap action”.
The advantages of detectors based on bistable magnetic behaviour in which the tag is based on magnetic wires can clearly be deduced from the results obtained with the latter type of materials, but the great length of the tag is a great drawback.
In addition to the advantages obtained with the tag in U.S. Pat. No. 4,660,025 which refer to its high harmonic content and its high pulse, it is important to find the possibility of deactivating this type of magnetic materials. U.S. Pat. No. 4,686,516 shows a way of doing this by the crystallization of the amorphous magnetic material. This is done by heating at least one part of the tag to a temperature that exceeds its crystallization temperature, by applying an electric current or a radiant energy such as a laser. Although some of the methods herein set forth allow deactivating the tag without touching it, they need to be cautiously applied.
U.S. Pat. No. 4,980,670 discloses a magnetic marker for the electronic surveillance of items in which the tag has “snap action” for low threshold values of the applied magnetic field, and, moreover, the tag is easily deactivated. This patent includes a method for manufacturing the tag based on magnetic films, the development of a detector and of a deactivator.
The conditions described in this patent for obtaining amorphous tapes with a bistable magnetic behaviour in the hysteresis cycle are based on special heat treatments of amorphous magnetic tapes to achieve the joining of magnetic domain walls. A certain number of compositions based on CoFeSiB, as well as treatment temperatures and times, are described in this patent.
The deactivation of this tag is carried out by subjecting the tag to a high-frequency and high amplitude alternating magnetic field. In this way, a great number of magnetic domains are created in the tape. The appearance of these domains in the tape avoids a Barkhausen jump in the hysteresis cycle, which makes the tag useless.
U.S. Pat. No. 5,313,192 discloses a tag that is equivalent to the one in U.S. Pat. No. 4,980,670, but more stable and controllable. The conditions for processing the amorphous magnetic tape are the same but the tag is also subjected to predetermined magnetic fields during the processing, which allow its activation and deactivation. More particularly, the tag of this invention contains a soft magnetic material forming the principal core, and a second hard or semi-hard magnetic material. This tag is conditioned in such a way that the second material has activated and deactivated states, respectively. In the activated state, the tag exhibits bistable hysteresis, whereas in deactivated state the tag has a hysteresis cycle without Barkhausen jumps.
U.S. Pat. No. 6,747,559 refers to a permanent tag for the electronic detection of items based on magnetic wires with low coercive forces (less than 10 A/m) and high magnetic permeability (greater than 20000). The length of the microwire or microwires used is not greater than 32 mm. In this case, it is the high permeability which allows obtaining high order harmonics, and with a high amplitude, for sufficiently low applied field values, thus making the tag easy to distinguish.